Currently, typical power semiconductor devices, including devices such as transistors, diodes, power MOSFETs and insulated gate bipolar transistors (IGBTs), are fabricated with silicon (Si) semiconductor material. More recently, wide-bandgap materials (SiC, III-N, III-O, diamond) have been considered for power devices due to their superior properties. III-Nitride or III-N semiconductor devices, such as gallium nitride (GaN) devices, are now emerging as attractive candidates to carry large currents, support high voltages and provide very low on-resistance and fast switching times. Although high voltage III-N diodes, transistors and switches are beginning to be commercialized, further improvements are needed in order to improve the performance, efficiency, reliability and cost of these devices. The term device will be used in general for any transistor or switch or diode when there is no need to distinguish between them.
Cross-sectional views of a group-III polar lateral III-N device 100A and an N-Polar lateral III-N device 100B are illustrated in FIGS. 1A and 1B, respectively. Devices 100A and 100B each include a source contact 21, a drain contact 22, a gate contact 23, and access regions 82 and 83. As used herein, the “access regions” of a device refer to the two regions between the source and gate contacts, and between the gate and drain contacts of the device, i.e., regions 82 and 83, respectively, in FIGS. 1A and 1B. Region 82, the access region on the source side of the gate, is typically referred to as the source side access region, and region 83, the access region on the drain side of the gate, is typically referred to as the drain side access region. As used herein, the “gate region” 81 of a device refers to the portion of the transistor between the two access regions 82 and 83 in FIGS. 1A and 1B. The gate module of the device refers to the portion of the layers and materials of the device that are in or adjacent to the gate region of the device, and within which the electric field is modulated through application of gate voltages in order to modulate the channel conductivity in the gate region of the device. The device channel refers to the conductive region that serves as the current path of the device between the source contact and drain contact when the device is biased in the ON state. The source contact 21 and the drain contact 22 are electrically connected to a lateral two-dimensional electron gas (2DEG) channel 19 (indicated by the dashed line in FIG. 1A) which is induced in a III-N channel layer 16 adjacent the interface between a III-N barrier layer 14 and the III-N channel layer 16 and serves as the device channel. The device channel in the gate region 81 of the devices of FIGS. 1A and 1 B is formed in a lateral direction from the portion of the 2DEG channel below the gate contact 23.
Typical III-N high electron mobility transistors (HEMTs) and related devices are formed on III-Nitride materials grown in a group-III polar (e.g., Ga-polar) orientation, such as the [0 0 0 1] (C-plane) orientation, as shown in FIG. 1A. That is, the source, gate, and drain contacts of the HEMT are formed over the group-III face (e.g., [0 0 0 1] face) of the III-N material layers, which is typically on an opposite side of the III-N material layers from the substrate on which the III-N layers are formed. Alternatively, III-N HEMTs can be formed on III-Nitride materials grown in an N-Polar (i.e., N-face) orientation, such as the [0 0 0−1] orientation, as shown in FIG. 1B. In this case, the source, gate, and drain contacts of the HEMT are formed over the N-face (e.g., [0 0 0−1] face) of the III-N material layers. N-polar III-N materials have polarization fields with opposite direction than group-III polar III-N materials, thus can enable the implementation of III-N devices which cannot be fabricated using group-III polar structures. N-polar III-N devices can in some cases exhibit superior characteristics when compared to group-III polar devices, including lower static and dynamic on-resistance, with higher current density, higher power density, and higher reliability.
Furthermore, III-N HEMTs are typically depletion-mode (D-mode) devices, which means they are normally-on, i.e., they conduct current when zero voltage is applied to the gate relative to the source and a positive voltage is applied to the drain relative to the source. However, in power electronics, it is more desirable to have normally-off devices, called enhancement mode (E-mode) devices, which do not conduct substantial current at zero gate voltage and require a sufficiently positive voltage applied to the gate relative to the source in order to be turned on. In power electronics, the use of E-mode devices can help to increase safety and to reduce the potential for damage to the device, to other circuit components, or to the entire power system by preventing accidental turn on of the device in case of circuit failure. However, improvements in the electrical performance of E-mode devices are still needed to further increase market adaptation.